One expert said the smartphone was unlikely to become a bestseller but was impressive nonetheless.

"Royole gets the bragging rights to being first, and it's quite astonishing that someone you've never heard of is doing this," said Carolina Milanesi, from the Creative Strategies consultancy.

"What's great is that it's putting this into the hands of developers, who will be able to start the legwork that will result in apps for flexible devices that will eventually be sold by Samsung and whoever else.

"You need developers to think through how they can best take advantage of screens that double in size."

She added that Royole might ultimately become an acquisition target for one of the mainstream consumer electronics brands.